With the recent improvements in road networks and vehicles' performance, high running performance is required even in heavy duty tires for trucks, buses and the like. In recent years, radial carcass and heavy duty radial tires in which the outside of the radial carcass is tightened by a rigid belt are widely employed. Such heavy duty radial tires are high in the rigidity of the tread portion and excellent in high speed performance, and further they show an excellent wear resistance, mileage and the like.
By the way, during running, a heavy duty radial tire is repeatedly subjected to the large stress in a region from the sidewall portion to bead portion. In this region, very small cracks (hereinafter referred "cracks" simply) are liable to occur due to such stress and ozone in the atmosphere.
The cracks make it impossible to recycle the tires, and there is a possibility that a carcass ply separation is caused if cracks are grown inwardly of the tire.
It becomes clear that, in heavy duty radial tires, especially those which are mounted on a five-degree-taper rim j of which bead seats j1 are tapered at five degrees with respect to the tire axial direction as shown in FIG. 12, since the outer surface of the bead portions comes into contact with a curved surface j2 of the rim flange over a wide range (for example 80% or more), a large strain is produced and heat generation occurs in the contacting part during running.
Such strain and heat, by their multiplier effect, harden or deteriorate the rubber in the bead portions which causes early damages such as cracks, carcass ply separation and the like.